This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The focus of the research proposed herein, a renewal of CHE030089N, is to apply modern quantum chemical methods to the elucidation of molecular mechanisms of organic chemical reactions that are used in the synthesis and biosynthesis of polycyclic organic molecules. During this award period, we will concentrate on the two subprojects shown in Figure 1: project 1 (continued from the previous grant period) cation-promoted polycyclization reactions involved in biosyntheses of terpene natural products;project 2 (continued from the related project from the previous grant period) pericyclic reactions used in the synthesis of complex cyclic molecules.